1. Field of the Invention
This invention relates to a process for thermal management in high temperature fuel cell systems by feed gas conditioning to result in higher overall system efficiency in the production of electricity. The process of this invention is advantageously utilized in the integration of gasification of naturally occurring carbonaceous material, such as coal, with molten alkali metal carbonates electrolyte fuel cell electrical production.
2. Description of Related Art
The catalytic reduction of carbon oxides to form methane is generally known, such as by the teachings of U.S. Pat. Nos. 2,465,462 and 4,242,103. U.S. Pat. No. 4,064,156 teaches production of methane wherein prior to methanation the feed gas is over shifted with CO.sub.2 removal to moderate temperature rise in the methanation reactors.
U.S. Pat. No. 4,569,890 teaches use of a low temperature phosphoric acid fuel cell, to consume hydrogen and to provide a higher methane content product gas in a coal gasification/fuel cell co-generation system.
Increasing the hydrogen content of the fuel feed stream to the anode compartment of a fuel cell is taught by several patents. U.S. Pat. No. 3,266,938 teaches a plurality of high temperature fuel cells arranged in series such that the spent gases from the anode compartment of the first fuel cell in the series is catalytically reformed exterior to the cell by an endothermic reforming reaction to produce additional hydrogen and then passed to the anode compartment of a second cell in the series; the spent gases of the anode compartment of the second fuel cell is passed to a catalytic exothermic shift reaction exterior to the cell for further production of hydrogen for passage to the anode compartment of a third fuel cell in the series. The reforming and shift reactions are performed exterior to the fuel cells to provide greater hydrogen content to the fuel feeds to the anode compartments of the fuel cells. U.S. Pat. No. 4,522,894 teaches increasing the hydrogen content of a liquid hydrocarbon feed by catalytic oxidation and steam reforming wherein use of thermal energy from the oxidation is used for reforming external to the fuel cell to produce high hydrogen content in the fuel feed stream to the anode compartment of the fuel cell. U.S. Pat. No. 3,488,226 teaches low temperature, low pressure steam reforming of liquid hydrocarbons to enhance hydrogen in the fuel feed for the anode compartment of molten carbonate fuel cells wherein the reforming is performed exterior to the fuel cell and acts as a heat sink for fuel cell produced heat. In one embodiment, the reforming catalyst may be placed in the fuel cell anode chamber. In either arrangement, the waste heat from the fuel cell is used directly to sustain the endothermic reforming reaction for the generation of hydrogen.
Reforming of hydrocarbonaceous fuels in separated compartments spaced within a fuel cell stack has been described in allowed U.S. Pat. No. 5,077,148, owned by the same assignee as this application.
In molten carbonates electrolyte fuel cell operation, a large percentage of the energy of the fuel is released as heat and must be removed from the fuel cell stack or system. At the operating temperature of molten carbonates electrolyte fuel cells of about 1200.degree. F., the heating value of hydrogen is equivalent to 1.285 volts. In normal operation of such a fuel cell, at about 0.70 to about 0.75 volt output, about 55 to about 65 percent of the energy content of hydrogen fuel is recovered as electricity and the remainder is released as heat which must be removed from the system. However, in systems where gas derived from fossil fuels is used, carbon monoxide having a heating value equivalent to 1.5 volts cannot be utilized directly in the electrochemical reaction and undergoes a water-gas-shift reaction in the anode compartment of the fuel cell producing additional hydrogen fuel, but releasing heat as the carbon monoxide is reacted. This additional heat must also be removed from the cell, and in such fuel cell systems the relationship of electrical energy to heat energy is less than stated above. In present day molten alkali metal carbonates electrolyte fuel cell operation, the internally produced heat is withdrawn with the process gases by allowing the temperature to rise across the cell. In molten alkali metal carbonates electrolyte fuel cell operation where anode gases are passed countercurrent to cathode gases, the heat is removed by the temperature rise of the cathode process stream where the generally permissible temperature rise is about 180.degree. F. To approach the desired operating temperature of about 1200.degree. F., the cathode inlet temperature must be maintained at about 1110.degree. F. and the outlet temperature about 1290.degree. F. The cathode feed stream comprises oxidant, usually air, and combusted anode exhaust to provide the necessary recycle volume of carbon dioxide within the system. This quantity of flow through the cathode is not high enough to withdraw sufficient heat from the system to maintain the permissible temperature rise. Generally, recycle of the cathode exhaust stream to the cathode inlet after cooling is used to provide sufficient mass to withdraw heat from the system to maintain the desired temperature. However, this procedure has several severe disadvantages: dilution of the cathode feed with lean cathode exhaust results in low concentration of cathode reactants thereby reducing the cell voltage; pressure drops across the cathode are increased due to the relatively high gas flow; and a large recycle blower is required due to the high recycle flow necessary and the high temperature of the cathode exhaust. It can be estimated that in atmospheric pressure systems, as may be required by the economics of small, on-site power plants, the power required for the cathode recycle blower can be in the order of 12.3 percent of the total energy fed to the system in the initial fuel gas. In such a heat-balanced system, the recycle ratio of the quantity of cathode exhaust recycled to the quantity of final system exhaust is about 4.4. With this quantity of cathode recycle, the cathode feed concentration of carbon dioxide is 7.94 percent and of oxygen is 7.37 percent, based upon an overall stoichiometric air to fuel ratio of 1.6. These concentrations are significantly lower than the 30 percent carbon dioxide and 15 per oxygen normally encountered in laboratory fuel cells, causing significant voltage loss in the system output.